OpenERP does not set language attribute. The best method is to create a web module to set language attribute on &lt;html&gt; tag and use attribute or :lang selector on your styles. That way, you can add css normally and it will be effective only when the target language is used.

thank you Mohamed ,can you please explain more , I can change the user interface to be rtl ( the side menu will be in the right) by adding (direction: rtl;) to css class (openerp) in base.css file (addons\web\static\src\css\base.css),

but I want this to work with change lang selection from english and arabic and vs